Imagine the Universe News - 10 April 2003

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The Hole Story?
New Type of Black Hole Poses Challenges

10 April 2003

A half-century ago, black holes were thought of as little more than
bizarre, hypothetical ideas arising from some of Einstein's equations taken
to their extreme. Today, black holes are accepted by astronomers
as a basic component of the Universe.� Under the "standard model" of
black holes, they come in two classes: the stellar and the supermassive.

And now, thanks to the efforts of two teams of scientists working
independently, there may be a new, third class: the intermediate mass.�
Although the case for their existence dates back several years, the new
findings provide much stronger, although not yet conclusive, proof that
such a class of black holes really exists. The only problem is, the
standard model just can't account for such a type of black hole.

The stellar class includes what is now viewed as the garden-variety black
hole, thought to be strewn throughout most, if not all, galaxies. Such a
black hole forms when the especially heavy core of a dying star collapses
onto itself under its own crushing gravity. These black holes have
masses anywhere from three to ten times the mass of our Sun -- but compressed
down to a singular point. (Because no information can escape a black hole
beyond its event horizon, the exact nature of this "singularity" is
unknown.)

Supermassive black holes (SMBHs) exist at the very center of many
galaxies, including our own Milky Way Galaxy. SMBHs usually weigh in on
the order of millions or billions of times that of our Sun. How such
monsters have developed isn't completely clear but astronomers now
believe SMBHs and their host galaxies probably "co-evolved" together over
time.

But what about intermediate mass black holes?

Defined loosely as having a mass anywhere from 100 to 1000 times that of
the Sun, intermediate mass black holes (IMBHs) theoretically shouldn't
exist because there is no really good way to create them. No single star
could ever be large enough to create such a heavy black hole. And while
evidence suggests that colliding galaxies result in coalescing SMBHs,
random stellar black hole mergers are probably too rare to produce IMBHs.

Using a pair of X-ray telescopes,
Dr. Tod Strohmayer and Dr. Richard Mushotzky at the NASA Goddard Space
Flight Center in Greenbelt, Md., and a team led by Dr. Jon Miller at the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.,
determined the masses of three suspected intermediate black holes.�

Miller's team measured the temperature of the superheated gas believed to
make up the swirling accretion disk of material being pulled
in by two of the black holes. Then by utilizing the known inverse
relationship between the mass of a black hole and the temperature and
velocity of the gas at different locations within the disk, they
estimated the masses of these two black holes between 200 and 500 solar
masses, making them IMBHs.

"Evidence is mounting that these elusive intermediate mass black holes
may really exist," Miller said. "The mystery, really, is how they can
exist."

The Strohmayer and Mushotzky team measured variations in the energy
being emitted by the accretion disk surrounding the third black hole
(located in a separate galaxy).� In addition to visiblelight, accretion
disks are so hot they emit lots of X rays. The two wanted to know if this
black hole appeared so "bright" in X rays because its orientation to the
Earth is such that all the X rays are "beamed" in our direction. The
effect is similar to a shining a flashlight in someone's eyes, except in
X rays.� This "brightness" would make the stellar-mass black hole seem
much heavier than it actually is. They concluded it is highly unlikely
all the X rays from the accretion disk would be beamed toward the Earth.�

The two also examined the X-ray emissions from iron atoms in the
accretion disk.� This is because X-ray emission from iron atoms look
different if the atoms are moving extremely fast. Fast moving atoms
occur in the accretion disk due to the pull of the black
hole. Specifically, the emission
lines appear wider (broader). The presence of the "broad iron line" is a
telltale sign of a black hole.

With both measurements, the Goddard team estimated the black hole's mass
to be at least 50 times greater than a stellar one, making it an
intermediate mass candidate.

Both studies necessarily relied on indirect evidence. Future studies may
provide more evidence or answers. And if it turns out IMBHs do in fact
exist, the standard model of black hole formation will have to be
rewritten to include them.